I remember reading how they worked out the horsepower for a traction engine which is totally different to the horsepower of a car. Basically you take the diameter of the cylinder and the stroke, multiply them together and divide by 10, then round it up or down. eg. an 8" diameter cylinder with an 8" stroke = 64 divide by 10 = 6.4 so the engine would be a 6hp engine. Applying that to our little engines with a 3 x 3.25 = 9.75 divide by 10 = 1hp. It gets a bit more complicated with a DCC.

Bear in mind this was only a guide to the power of the engine, we are so familiar with the power of this & the power of that being accurately measured now. 150 years ago the horse reigned supreme for most sources of power and as far as engines went they just wanted to know "would it do the job or not?"

I guess your right on that count. If one our little ol 4" agricultuaral engines can pull a 2 ton truck then I reackon that's power enough to do most jobs at that scale!!Looks like "on average" the answer to the question is "around one & a half horses"!!!Perhaps STW could give a definative answer............... Well Craig!!!.......... RegardsTony

I can't find the original article regarding horsepower but I have found an article in the Burrell Style book which relates solely to Burrells and effectively gives a similar result. I've reproduced it below:

The reference to engine size by means of nominal horse power rating had been used since the earliest days and for single cylinder engines was calculated on the diameter of the bore, the piston stroke not being taken into account. Virtually all makers followed the system using the formula NHP= DxD/10 where D is the cylinder diameter in inches. Thus the following standard dimensions were established and used to the last days:-NHP Diameter Stroke5 7" 10"6 8" 10"7 8.5" 12"8 9" 12"Steam pressure and piston speed were ignored but generally these factors were fairly consistant such that the variation in output between the different makers did not amount to very much. The exception was the use on the late Burrell engines of a working steam pressure of 200psi. which raised the power for a given rating above others.The introduction of the compound system meant that a different formula was required and exactly what this was has never been established, makers generally following their own ideas on the subject. One assumption that could be made was that, since under ideal conditions equal work was done in high and low pressure sides, one half of the total NHP was to be derived from the high pressure cylinder thus.1/2 NHP = DxD/10 where D is the diameter of the high pressure piston.or NHP = DxD/5The diameter chosen for the low pressure piston reflected the total number of expansions intended, bearing in mind the cut off in the high pressure and the desire for equal output from each cylinder under normal conditions of work. Usually the low pressure piston area was about 2.8 or 2.9 times that of the high pressure and calculating accordingly the hypothetical dimensions could become as follows:NHP Dia HP Dia LP5 5" 8.5"6 5.5" 9.25"7 6" 10"8 6.25" 10.5"10 7" 12"Again the rating ignored stroke, speed and steam pressure but gave a guide to size with an actual output equal to or a little above that for a single engine.

So for the SC with a diameter of 2.75" 2.75x2.75/10 = 0.75 NHP

So for the DCC with a high pressure diameter of 2.28" 2.28x2.28/5 = 1.039 so a fraction over 1 NHP

So by these figures the DCC will have just over 33% more power than the single crank engine